The present invention pertains generally to the manufacture of cathode ray tubes. It is specifically directed toward improved apparatus for depositing phosphor or other material coatings onto the front panels of such tubes.
Picture tubes of the type with which this invention is concerned consist of two separate units, a front panel assembly and a funnel assembly, each manufactured and processed separately, which are ultimately united. In conventional color picture tubes, the front panel assembly includes a glass front panel having a 2 to 3 inch glass skirt around its perimeter and an aperture mask assembly comprising an aperture (shadow) mask mounted on and supported by a steel frame which is in turn supported and positioned within the front panel by studs extending inwardly from the panel skirt. A glass funnel which comprises part of the funnel assembly mates with the front panel and is frit-sealed thereto.
One phase of the manufacture of cathode ray tubes, both black-and-white and color, involves depositing one or more phosphor coatings onto the front panels of such tubes. In the case of color tubes, successive coatings of green-emitting, blue-emitting and red-emitting phosphor materials are applied to each front panel. A most common way of applying such phosphor coatings on cathode ray tube (CRT) front panels has been to dispense a phosphor slurry into an inverted front panel. The panel is then tilted and spun in a manner well known in the art in order to provide a uniform slurry coating over the inner surface of the panel. The panel is then tilted further in order to dump any excess slurry.
This "dispense and spin" method, although subject to certain disadvantages, has found considerable application in the commercial production of color CRT's. While this method is reasonably well suited for processing conventional skirted panels, it is not well suited for processing a new type of CRT front panel which has no skirt around its perimeter. A fuller discussion of such new panels and the reasons why the dispense and spin method of phosphor coating is unsuitable for them will be deferred until certain problems which are associated with the dispense and spin method have been examined. This will help to clarify the objectives of the present invention and to illuminate the particular problems to which the invention is addressed.
One problem which contributes to the high cost of manufacturing CRT's is that the dispense and spin method, as practiced in the manner described above, is a relatively slow process. The total time required for the application of one coating is in the order of one or two minutes. In color tube screening, the process must be repeated for each of the three phosphor materials, resulting in a total elapsed time for the application of the phosphor coatings, exclusive of the time required for exposure, developing and drying, of 5 minutes or more.
Another problem which exists in the above-described dispense and spin method of phosphor coating is that any contaminants which are found on the panel will be mixed in with the dispensed slurry and may find their way back into the phosphor source when the excess slurry is dumped and reclaimed. Since this reclaimed slurry is generally reused, those contaminants can be undesirably recycled back onto another panel where their presence may cause pock marks or other disturbances in the coating. Such irregularities in phosphor coatings can cause a significant drop in the production yield factor, thus adding to the per unit cost of the finished product.
A third disadvantage which is inherent in the dispense and spin procedure is that the excess slurry which is dumped and reclaimed has a phosphor content which is less than that of an unused slurry. This is because the phosphor particles which are suspended within the slurry tend to settle onto the panel surface. A certain amount of phosphor material must, therefore, be added to the reclaimed slurry before it can be reused. The added step of restoring the reclaimed slurry to its former state further complicates an already complex process. In addition to complicating the entire process, this step is subject to erros which can adversely affect product uniformity and yield.
A fourth problem with this method of applying the phosphor coating is that during the coating of the panel, the larger phosphor particles tend to settle our first. When the excess slurry is dumped, it will contain fewer of the large phosphor particles than an unused slurry and, if mixed directly with that unused slurry, may cause subsequent panels to receive coatings which do not have the desired phosphor particle distribution. This problem, along with the one discussed immediately above, tend to cause long term process variations which result in nonuniform phosphor weight distributions in panel coatings. Such variations are particularly evident between panels processed soon after production start-up and those processed later.
This invention is directed toward apparatus well suited for the application of coatings of phosphor and other materials onto skirtless CRT panels while avoiding the above-described problems which are associated with the application of phosphor coatings onto conventional skirted CRT panels.
______________________________________ Prior Art PATENT NUMBER ISSUED TO ______________________________________ 771,443 Perkiewicz 1,200,065 Yingling 2,745,419 Slingerland 2,916,012 Hergenrother 3,132,968 Wandtke 3,242,003 Brown 3,341,354 Woods et al 3,345,933 Glaus 3,345,973 Glaus 3,364,055 Nelson 3,365,325 Fraenkel et al 3,526,535 Plumat ______________________________________
Curtain Coating Picture Frames, by Ward, D. March, 1960, Wilco Machine Works, Inc., Memphis, Tenn. Reprinted from Hitchcock's Wood Working Dig., December, 1959.